Ignition control system



Feb. 7, 1950 R. STEENSMA IGNITION CONTROL SYSTEM Filed March 11, 1947 INVENTOR 7 RI E H ARD ETEENEM A.

ATTORNEY TORILAY 44 Patented Feb. 7, 1950 IGNITION CONTROL SYSTEM Richard Steensma, Fair Lawn, N. J., assignor to Wright Aeronautical Corporation, a corporation of New York Application March 11, 1947, Serial No. 733,805

8 Claims.

This invention relates toignition control systerm for burners and is more particularly directedto an ignition controlsystem for a burner in which theburner igniter means is automatically rendered operative upon extinction of flame.

Gasturbine and jet" engines operate with a the burner large amount of exces'sair which flows over the supply. Thiscombustion difllculty is particularly present in engines designed for aircraft use because such engines operate under a wide range of conditions which may change quite suddenly.

'A primary object of the invention comprises the provision of means associated with a burner for automatically reigniting the combustion mixture should the combustion flame become extinguished. Although the invention is'of general application, it has been designed for use with aircraft engines and, accordingly, the system preferably-should be'si'mple, quite rugged and capable of operating at high temperatures.

Specifically the invention comprises an ignition control system including one or more pairs of spaced electrodes disposed in the path of the combustion flame, the increase in impedance across said electrodes, upon extinction of the combustion flame, beingeflective to render suitable burner igniter means operative as long as said flame remains extinguished; When said flame is re-established, the decrease in impedance across said electrodes isefiective to render said igniter means inoperative.

Other objects of the invention will become apparent upon reading the annexed detailedde scription in connection with the drawing in which:

Figure 1 is a diagrammatic view disclosing the combustion igniter control means of'the presen invention; and

Figure '2 isa partial view of a modified'form the invention. I I

Referring first to Figure 1 of the drawing, a

combustion chamber l'fl'i's' provided with a burner zlei In order to ignite the fuel and air, one or more spark plugs 18 are disposed in the flow path of the combustion mixture. For this purpose, and as hereinafter described, the spark plug or plugs H] are connected in a circuit which, when closed, produces a series of sparks across the electrodes of the plug i8 for igniting the combustion mixture, said sparks continuing until combustion has been established or re-established. As will appear, the specific construction of the combustion chamber, the burner and the igniter therefor form no part of the present invention.

One or more pairs of spaced electrodes 20 and 22 are disposed in the combustion chamber ill for detecting the presence or absence of the combustion flame therein. These electrodes may comprise the electrodes of a conventional spark plug. Preferably, however, one of the electrodes 20 comprises a central insulated electrodeof a plug-like member 24 while the adjacent grounded wall of the combustion chamber ID or the grounded outer sleeve of the plug-like member 24 forms the other electrode 22. The distance between the electrodes 20 and 22 of the flame detectors 24 is not critical; however, it is essential that the gap between these electrodes lie in the path of the burner flame 23. The flame detecting devices 24 are disposed at different locations relative to the burner nozzle in order that the electrodes of at least one said device 24 are disposed in the burner flame regardless of the length of'theflame. For example, with two flame detectin devices 24, the one is disposed relatively close to the burner nozzle in order to detect a short flame at low rates of fuel supply, but this flame detector may not be in the path of the flame at high rates of fuel flow. Accordingly, the

other flame detector 24 is disposed at some dis'-' tance from the burner nozzle in order to detect a relatively long burner flame, as exists at high rates of fuel supply.

The ignition circuit for the spark plug I8 is conventional. I is connected across the high tension winding of a transformer 30 while the low tension winding of said transformer is connected in-series with conventional .vibrator means 32, a battery 34, a switch 35' and a small resistance 36 to a pair of contacts 38. The contacts 38 are controlled by a switch arm 42 of a relay 44. The relay arm 42 is biased to its position bridging the contacts 38, for example by a spring'45. A condenser to is con. nected across the contacts 3Bto minimize arcing when the switch arm 42-breaks the ignition circuit at said contacts. The particular form of'igniter for the burner l2 forms no part of" the pres As illustrated, the spark plug it.

ant invention, in fact, as mentioned, the spark plug igniter l8 and the ignition circuit therefor is conventional. The invention is primarily directed to means for controlling the relay 44 or an equivalent device so as to automatically render the combustion igniter means operative when the combustion flame is extinguished. Accordingly, the provision of any suitable igniter under the control of the relay 44 or under the control of an equivalent device is within the scope of the invention.

With the ignition circuit and igniter means illustrated, after the switch 35 has been closed and when the relay switch arm 42 bridges the contacts 38, the vibrator 32 operates to make and break the primary circuit of the transformer 30 whereby as long as the switches 35 and 42 remain closed, a series of sparks is produced across the electrodes of the igniter plug l8 for igniting the combustion mixture. By opening the switch 35 the operator can render the igniter inoperative when the burner is shut down. An indicator lamp 45 is connected across the resistance 86 whereby the lamp 45 is energized whenever the igniter is operative thereby informing the opera tor of this fact. In addition means may be provided so that the operator can render the igniter operative independently of the relay 44. For this purpose a normally open switch 48 may be connected in parallel with said relay.

The relay 44 and the control apparatus therefor preferably is housed in a grounded metal container schematically indicated at 41. An alternating current source of electric power 48 is supplied to said control apparatus through power terminals 50 and 52 and a switch 54, the terminal 50 and the metal container 41 being grounded as indicated at 56. A pair of resistances 58 and 60 are serially connected across the terminals 50 and 52 to form two arms of a bridge circuit. A resistance 62 forms the third arm of said bridge circuit and the flame detecting electrodes 20 and 22 are included in the fourth arm of said bridge circuit. The outer end 63 of each insulated electrode 20 is connected to a wire lead 64 which leads are joined to a common lead 65 extending into the container 4'! for connection to one end of the resistance 52 preferably through a rectifier 65. For this purpose the wire lead 65 is connected to the plates or anodes 68 of the rectifier tube 68 and a wire 10 connects the cathodes 12 of the tube 68 to one end of the resistance 62. The return circuit to the electrode 22 comprises a ground return through a metallic shield 13 disposed about the wire leads 64 and 65. The shield 13 is connected at one end to the grounded metal container 4? and at the other end to the grounded metal wall of the combustion chamber l forming the grounded electrodes 22. The other end of the re sistance 82 is connected to a contact 14 adjustable along the resistance 60.

With this bridge circuit construction the electrodes 20 and 22 with their rectifier 66 comprise one arm of said circuit, and the resistances 58, 80 and 62 comprise the other arms of said circuit. A condenser I is connected in parallel with the resistance 52 to balance the electric capacitance between the wire leads 64 and 65 and the grounded shields 13, which capacitance is in parallel with the gaps between the pairs of electrodes 20 and 22.

A thyratron tube 16 is connected across the bridge circuit so that said tube fires, that is. becomes conducting, when the resistance across at least one pair of electrodes 20 and 22 is reduced by the presence of a flame therebetween.

For this purpose the control grid 18 of the thyratron tube 18 is connected by a wire 80 to the point 8| between the bridge arm comprising the resistance 62 and the bridge arm comprising the flame detecting electrodes 20 and 22 with their rectifier 66. The cathode 82 of the thyratron tube 16 is connected by a wire 84 to the junction 88 of the resistances 58 and 80; and the thyra tron tube is provided witha" shield grid 88 connected to said cathode by a Wire 90. The filaments or heaters 92 and 94 of the tubes 18 and 68 respectively, are connected by wires 84, 96 and 98 across the secondary or low tension winding of a transformer I00, the primary of which is c0nnected across the terminals 50 and 52 by wires I02 and H14. The anode or plate 108 of the thyratron tube 16 is connected to the terminal 50 through the relay 44', a wire I08 and the wire I02 whereby the relay 44 is energized to open the ignition circuit at contact$138 when the thyratron tube becomes conducting. With, the aforedescribed arrangement, the control grid 18 and its cathode 82 are connected across the bridge circuit at the points 8! and 86 whereby the bridge circuit controls the relative electrical potential of said control grid and cathode. In addition the thyratron or gas tube 18 can be rendered conducting only when the power terminal 50' is at a positive electric potential relative to the terminal 52. During the portions of the alternating current cycle when the power terminal 50 is positive, the tube 15 is rendered conducting only when the electric potential of its control grid 18 is above a predetermined value relative to its cathode. Also during said cycle portions when the terminal50 is positive, if there is a flame in the combustion chamber in at either pair of flame detecting electrodes 20 and 22, the impedance across said pair or pairs of electrodes 20 and 22 will be relatively small so that the electric potential of the control grid 18. will at least be less negative as compared to its electric potential when the impedance across both pairs of said electrodes 20 and 22 is relatively high because of the absence of a flame. Accordingly, the sizes of the condensers and resistances of the bridge circuit may be so selectedand the contact 14 so adjusted that the thyratrontube 16 is automatically rendered conducting during that portion of each cycle of the alternating current source 48 when the terminal 50 is positive and when there is a flame at either pair of flame detecting electrodes 20 and 22, and said tube is automatically rendered non-conducting in the absence of a flame at both pairs of said flame detecting electrodes. With a volt alternating current power source 48, a No. 6H6rectifler tube 66 and a No. 2D21 thyratron tube 16, the system operated satisfactorily with resistances 58 and 60 equal to 45,000 and 1000 ohms, respectively, resistance 62 equal to -five meg-ohms and condenser 15 equal to .01 microfaradi Obviously, however, the specific circuit, its voltage and its tubes, as well as the specific magnitude. of its resistances and capacitances are subject to considerablevariation. The large decrease in-impedance.across the gap between each pair of electrodes 20 and 22 in the presence of a flame is believed to be due to ionization existingin a'flame- It is a characteristic of thyratronor gas tubes that once the tube becomes conducting, the tube remains conducting and the grid loses control until the tube platevoltage falls. below that required to maintain ionization within the tube 16.

.If the plate voltage ofla thyratron tube is periodically decreased s-ufllciently to render the tube non-conducting for a short length of time its grid immediately regains control each timethe plate voltage is so reduced. For this reason, the plate circuit of the thyratron tube 15 is connected to an alternating current power source 48. With this arrangement, during each portion of the alternating current cycle that the power terminal 50 is positive, the .plate voltage of the tube 16 will be above that required to maintain the tube conducting so that if the potential of the control grid 18 is sufiiciently high during said cycle :portions, the tube 16 will be conducting during each said cycle portion. Accordingly, as long as a combustion flame continues to exist in the combustion chamber ID, the tube 16 will fire (become conducting), during each cycle portion that the power terminal 50 is positive, thereby rendering the igniter l8 inoperative by energizing the relay 44 to open the contacts 38.

If the combustion flame becomes extinguished for any reason, the tube 16 will become non-conducting during the next cycle portion that the power terminal 50 is negative and the tube will remain non-conducting as long as the combustion flame is extinguished thereby de-energizing the relay 44 whereupon the relay switch arm 42 bridges its switch contacts 38 to render the igniter l8 orperative (switch 35 having been closed in starting the burner). Thus, a series of sparks is roduced by the igniter plug 18 until a flame has been established in the combustion chamber 10 at which time the relay 44 is again energized as described to open the ignition circuit at the contacts 38 thereby rendering the igniter I 8 inoperative. Similarly, in starting the burner, when the switches 35 and 54 are closed, a series of sparks is produced by the igniter plug I8 until a combustion flame has been established. The output of the thyratron tube 16 is a pulsating unidirectional current so that the arm 42 of the relay 44 ma tend to chatter against its contacts when said relay is energized by this current. To prevent this possibility a condenser H is connected in parallel, with the relay 44.

Because of the grounded shielding 13 about the wire leads 64 and 65 there is considerable electric capacitance between said leads and shielding, which capacitance is in parallel with the electrodes 20 and 22. The impedance of this capacitance is quite low when an alternating current voltage is impressed across the electrodes 20 and 22 and therefore with such an alternating current voltage, the impedance of the arm of the bridge circuit including said electrodes would be quite low even in the absence of a flame between said electrodes. However, when the rectifier 66 is connected in series with the electrodes 20 and 22 a pulsating direct current voltage is applied across said electrodes, thereby greatly increasing the aforementioned impedance in parallel with the said electrodes. Accordingly with the rectifier tube 66 in series with the electrodes 20 and 22 there is a larger increase in the impedance in this arm of the bridge circuit when the flame between both pairs of electrodes 20 and 22 is extinguished as compared to the increase which would take place in the absence of said rectifier. Therefore, the presence of the rectifier 66 greatly increases the sensitivity of the bridge circuit. In fact without the rectifier 66 and with the resistance 62 equal to approximately megohms the thyratron tube It remainedconducting regardless electrodes 20 and 22. In addition because the im- 6. pedance of the capacitance between the wire leads 64 and 65 and the grounded shield I3 is large with the rectifier 66 connected into the bridge circuit, the bridge circuit control of the thyratron tube 16 is substantially independent of the length of said wire leads.

The rectifier tube 66 has been illustrated as a this arrangement, if some portion of the system should fail, the worst that probably would happen is that the relay 44 would become or remain deenergized whereby the igniter would be operative even though the combustion mixture may be burning in the chamber I0. Obviously, however, if desired the system may be reversed to the extent that the presence of a combustion flame would be effective to de-energize the relay 44 thereby rendering the igniter inoperative and upon extinction of the combustion flame the relay 44 would be energized to render the igniter operative.

As illustrated in Figure 1 two flame detecting devices 24 are disposed in the path of the flame from one burner l2 and these devices are connected in parallel whereby the igniter I8 is rendered operative only when there is no flame at both flame detecting devices 24. It also ma be desirable to connect a plurality of flame detecting devices in series. Such an arrangement is illustrated in Figure 2 wherein like but primed reference numerals have been used to design-ate parts corresponding to similar parts of Figure 1.

Figure 2 discloses a plurality of individual burners l2 which may be disposed in a common combustion chamber or as illustrated in separate combustion chambers It. In Figure 2 the combustion chambers ID" are illustrated in transverse section instead of in longitudinal section as in Figure 1. A flame detecting device 24' and an igniter plug l8 are provided for each burner 12. Each flame detecting device 24' differs from the corresponding device 24 of Figure 1 in that both electrodes 20 and 22 are insulated from the walls of their associated combustion chamber. Also as illustrated the gaps between said pairs of electrodes 20' and 22 are connected in series, instead of in parallel, to wires 65 and 13 leading to a control system similar to that of Figure 1'. In.

addition in Figure 2 the igniters I 8' are connected to individual step-up transformers 3i! controlled by a relay 44' of the control circuit. In Figure 2 since the flame detecting electrode gaps are connected in series, the thyratron tube of the control circuit will remain conducting only as long as a flame is present at both pairs of electrodes 20' and 22'. If the combustion flame of either burner l2 becomes extinguished, the combined impedance of the serially connected gaps Of the electrodes 20' and 22 will affect the bridge control circuit so as torender its thyratron tube nonconducting whereupon both igniters l8 are rendered operative until both burners l2 are again operating. One or more flame detecting devices may be connected in parallel with each device 24' in order that each burner 12 be provided with a plurality of parallel connected fiame. de-.

tecting devices arranged, as in Figure 1, so that at least one flame detecting device of each burner is in the path of the burner flame regardless of the magnitude of said flame.

The use of a thyratron tube in the control circuit has the advantage in that as soon as the tube is rendered conducting, suflicient current flows through the tube to operate the relay 44. This is so since it is a characteristic of a thyratron tube that its control grid acts as a trigger in controlling the firing of the tube. That is,'as soon as the electric potential of the thyratron control grid becomes sufficiently high a relatively large plate current flows through the tube, the magnitude of which is independent of the specific electric potential of the control grid. This advantage of a thyratron tube results from its gas atmosphere. Accordingly as used in the claims the term thyratron tube is intended to be broad enough to cover equivalent forms of gas filled tubes as distinguished from so-called vacuum tubes. However, it should be obvious that other forms of electronic tubes or other electric relays may be substituted for a thyratron tube.

As used in this specification and appended claims the term impedance is a general form meaning resistance to electric current flow regardless of the nature of the current.

While I have described my invention in detail in its present preferred embodiment, it will be obvious to those skilled in the art, after understanding my invention, that various changes and modifications may be made therein without departing from the spirit or scope thereof. I aim in the appended claims to cover all such modifications and changes.

I claim:

1. Flame detecting apparatus for a burner comprising a pair of spaced electrodes arranged to be disposed in the path of the burner flame; a conductor connected to one of said electrodes; a grounded metallic shield surrounding said conductor and connected to the other of said electrodes; a source of alternating current; a rectifier; an electric circuit including said source of alternating current, said rectifier, said conductor, said grounded shield and said electrodes with said conductor connected to one side of said source through said rectifier and with the other side of said source connected to g and an electric relay electrically connected to said circuit, said relay being operable in response to changes in the electric impedance across the gap between said electrodes such that said relay is operable to a first condition as a result of the presence of a flame at said electrodes and is operable to a second condition in the absence of said flame.

2. Flame detecting apparatus for a burner comprising a pair of spaced electrodes arranged to be disposed in the path of the burner flame; a pair of conductors each connected to one of said electrodes; an electric bridge circuit comprising four arms connected together to form a closed network with a terminal at the junction of each pair of said arms; an alternating current source of power connected to a pair of opposite terminals of said bridge; a rectifier connected in series with said electrodes and conductors at the end of one of said conductors remote from its electrode; said rectifier, conductors and electrodes constituting one arm of said bridge circuit; and a thyratron tube having its cathode connected to one of the other terminals of said bridge and having its control grid connected to the remaining terminal of said bridge such that said tube is responsive to changes in the electric impedance across said gap, as a result of the presence or absence of a flame at said electrodes, for detecting the presence or absence of said flame.

3. Flame detecting apparatus for a burner comprising two pairs of spaced electrodes, said two pairs of electrodes being electrically connected in parallel and being disposed at spaced points so that one pair of said two pairs of electrodes is disposed in the path of the burner flame when said flame is relatively short and the other pair of said two pairs of electrodes is disposed in the path of the burner flame when said flame is relatively long; an electric circuit connected to said parallel connected electrodes; and an electric relay operatively controlled by said circuit so as to be responsive to changes in the combined electric impedance of the parallel connected gaps between said pairs of electrodes, said relay being operable to a first condition as a result of the presence of a flame at either of said two pairs of electrodes and being operable to a second condition in the absence of a flame at both of said two pairs of electrodes.

4. Flame detecting apparatus for a burner comprising a pair of spaced electrodes arranged to be disposed in the path of the burner flame; a pair of conductors each connected to one of said electrodes; a source of alternating current; a rectifier; an electric circuit including said source of alternating current, said rectifier, said conductors and said electrodes with one of said conductors connected to one side of said source and with the rectifier interposed between the connection of the other of said conductors to the other side of said source; and an electric relay electrically connected to said circuit, said relay being operable in response to changes in the electric impedance in that portion of said circuit including said conductors and electrodes, as a result of the presence or absence of a flame at said electrodes, for detecting the presence or absence of said flame.

5. Flame detecting apparatus as recited in claim 4 in which one of said conductors constitutes a grounded shield for the other of said conductors.

6. Flame detecting apparatus as recited in claim 4 in which said electric circuit constitutes a bridge circuit with said rectifier, said electrodes and said conductors forming one arm of said bridge circuit.

7. Flame detecting apparatus for a burner comprising a pair of spaced electrodes arranged to be disposed in the path of the burner flame; a pair of side-by-side conductors each connected to one of said electrodes; a source of alternating current; a rectifier; an electric circuit including said source of alternating current, said rectifier, said conductors and said electrodes with one of said conductors connected to one side of said source and with the rectifier interposed between the connection of the other of said conductors and the other side of said source; and an electron tube electrically connected to said circuit such that variations in the electric impedance in that portion of said circuit including said conductors and electrodes, as a result of the presence or absence of a flame at said electrodes, varies the output of said tube for detecting the presence or absence of said flame.

8. Flame detecting apparatus for a burner comprising two pairs of spaced electrodes; a pair of conductors, said two pairs of electrodes being electrically connected in parallel to said conductors and being disposed at spaced points so that one pair of said two pairs of electrodes is disposed in the path of the burner flame when said flame is relatively short and the other pair of said two pairs of electrodes is disposed in the path of the burner flame when said flame is relatively long; a source of alternating current; a rectifier; an electric circuit including said source of alternating current, said rectifier, said conductors and said electrodes with one of said conductors connected to one side of said source and with the rectifier interposed between the connection of the other of said conductors to the other side of said source; and an electric relay electrically connected to said circuit, said relay being operable in response to changes in the electric impedance in that portion of said circuit including said conductors and said parallel connected pairs of electrodes such that said relay is operable to a first condition as a result of the presence of a flame at either of said two pairs of electrodes 10 and is operable to a second condition in the absence of a flame at both of said two pairs of electrodes.

RICHARD STEENSMA.

REFERENCES CITED The following references are of record in the file of this patent: 

